Electronic security systems are known for preventing unauthorized removal of articles from a zone under protection. Such systems are especially suitable for use in retail stores to prevent pilferage of articles and to minimize considerable losses occasioned by shoplifting. A particularly effective system is described in U.S. Pat. No. 3,810,147 and copending application Ser. No. 262,465, filed June 14, 1972, now U.S. Pat. No. 3,863,244 wherein a multi-frequency resonant tag having different frequencies for detection and deactivation is attached to articles of merchandise. The resonant tag circuit is operative at a first frequency to permit detection of tag presence in a protected zone by electromagnetic interrogation thereof and is operative at a second frequency to permit deactivation thereof by an applied electromagnetic field which destroys the resonant property of the circuit at its detection frequency. In order to prevent false alarms occasioned by the detection of spurious signals not originating from the presence of a resonant tag in a protected zone, it is necessary to distinguish true signals from noise in circumstances where the noise level may be greater in amplitude than the expected signals.
Copending application Ser. No. 262,465 describes signal discrimination circuitry which provides an output signal in response to received signals of predetermined amplitude, pulse width, pulse spacing, polarity and sequence, such that output pulses are provided only in response to true signals received from a resonant tag in the protected zone. These output pulses are in turn applied to noise rejection circuitry which responds to a predetermined pulse repetition rate for actuation of an alarm or other output utilization apparatus. Copending application Ser. No. 389,728, filed Aug. 20, 1973, now U.S. Pat. No. 3,828,337, describes particularly effective noise rejection circuitry which may be employed in an electronic security system. The present invention represents a refinement of the apparatus disclosed in copending application Ser. No. 262,465 and may be employed in an electronic security system in conjunction with the noise rejection circuitry described in copending application Ser. No. 389,728.